X-ray inspection of bumps on a semiconductor substrate

1. Inspection apparatus, comprising: an X-ray source, which is configured to emit a focused beam of X-rays toward a semiconductor wafer; a motion stage, which is configured to align a location of the focused beam relative to the wafer so that the beam is focused on a feature formed on the wafer, the feature including a volume containing a first material and a cap made of a second material, different from the first material, that is formed over the volume; an array of detectors, comprising first and second detectors positioned at opposing azimuthal angles relative to the feature, which are configured to detect X-ray fluorescent photons that are emitted by the first material in response to the irradiating beam and pass through the cap before striking the detectors, and to output respective first and second signals in response to the detected X-ray fluorescent photons; and processing circuitry, which is coupled to measure and normalize a difference between first and second signals and to identify, in response to the normalized difference between the signals, a void enclosed inside the cap.

2. The apparatus according to claim 1 , wherein the array of detectors further comprises third and fourth detectors, which are positioned at mutually-opposing azimuthal angles between the first and second detectors and are configured to output respective third and fourth signals in response to the X-ray fluorescent photons, and wherein the processing circuitry is coupled to measure a further difference between the third and fourth signals in order to identify the void.

3. The apparatus according to claim 1 , wherein the processing circuitry is configured to measure an attenuation by the second material of the X-rays emitted in a characteristic fluorescence line of the first material.

4. The apparatus according to claim 3 , wherein the processing circuitry is configured to estimate a size of the cap responsively to the attenuation.

5. The apparatus according to claim 1 , wherein the detectors and configured to detect further X-rays emitted from the cap in response to the irradiating beam in a characteristic fluorescence line of the second material, and wherein the processing circuitry is configured to assess a misalignment of the array relative to the feature based on a difference between the X-rays in the characteristic fluorescence line detected at the different angles, and to correct for the misalignment in detection of the X-ray fluorescent photons.

6. The apparatus according to claim 5 , wherein each of the detectors is configured to simultaneously detect the X-ray fluorescent photons that are emitted by the first material and the further X-rays emitted from the cap.

7. The apparatus according to claim 3 , wherein the volume comprises a pillar containing a first metallic element, and the cap includes a second metallic element, configured to serve as an electrical contact for the pillar.

8. A method for inspection, comprising: irradiating, with a focused beam, a semiconductor wafer; aligning the beam with the wafer so that the beam is focused on a feature formed on the wafer, the feature including a volume containing a first material and a cap made of a second material, different from the first material, that is formed over the volume; detecting, using one or more detectors positioned at opposing, first and second azimuthal angles relative to the feature, X-ray fluorescent photons that are emitted by the first material in response to the irradiating beam and pass through the cap before striking the detectors; and measuring and normalizing a difference between first and second signals output by the one or more detectors at the first and second azimuthal angles, respectively, in response to the detected X-ray fluorescent photons, and identifying, in response to the normalized difference between the signals, a void enclosed inside the cap.

9. The method according to claim 8 , and comprising: detecting, using the one or more detectors, the X-ray fluorescent photons that are emitted by the first material in response to the irradiating beam and pass through the cap at mutually-opposing third and fourth azimuthal angles, between the first and second azimuthal angles; and measuring a further difference between third and fourth signals output by the one or more detectors at the third and fourth azimuthal angles, respectively, in response to the detected X-ray fluorescent photons in order to identify the void.

10. The method according to claim 8 , and comprising measuring an attenuation by the second material of the X-rays emitted in a characteristic fluorescence line of the first material.

11. The method according to claim 10 , wherein measuring the attenuation comprises estimating a size of the cap responsively to the attenuation.

12. The method according to claim 8 , and comprising detecting, using an array of the detectors, further X-rays emitted from the cap in response to the irradiating beam in a characteristic fluorescence line of the second material, and assessing a misalignment of the array relative to the feature based on a difference between the X-rays in the characteristic fluorescence line detected at the different angles, and wherein detecting the X-ray fluorescent photons comprises correcting for the misalignment in detection of the X-ray fluorescent photons.

13. The method according to claim 12 , wherein detecting the further X-rays comprises simultaneously detecting, in each of the detectors in the array, the X-ray fluorescent photons that are emitted by the first material and the further X-rays emitted from the cap.

14. The method according to claim 8 , wherein the volume comprises a pillar containing a first metallic element, and the cap includes a second metallic element, configured to serve as an electrical contact for the pillar.